1. Field of the Invention
The present invention relates to aerobic wastewater treatment systems, particularly residential wastewater treatment systems and, more particularly, to the denitrification of treated wastewater from such aerobic wastewater treatment systems.
2. Description of Prior Art
Typical residential aerobic wastewater treatment systems comprise a pre-treatment vessel or tank, an aerobic digestion vessel or tank and a pump vessel or tank. In operation, the raw wastewater, which can be a mixture of so-called black water and grey water from a residence flows to the pre-treatment tank where the bulk of the solids settle out. The largely solids free water from the pre-treatment tank flows into the aerobic digestion tank where under the influence of an oxygen containing gas, the bacteria aerobically digests the organic solids carried over from the pre-treatment tank. Most aerobic digestion tanks (aeration tank) are comprised of a so-called aeration chamber and a clarifier chamber, digestion of the suspended and dissolved organic solids being conducted in the aeration chamber, substantially clarified water being removed from the clarifier chamber which then flows by gravity into the pump tank, a holding tank or in some cases for direct disposal.
Wastewater generally contains large amounts of nitrogen in the form of nitrates (NO3−), nitrites (NO2−), ammonia (NH4+) and nitrogen gas (N2). All these forms of nitrogen are biochemically interconvertible, the former three being nitrogen nutrients. Nitrogen nutrients from wastewater can lead to the nutrient enrichment of water bodies causing excessive growth of aquatic plants (algae). The dissolved oxygen in the water body becomes depleted when the aquatic plants die, fall to the bottom and then are decomposed by bacteria. The oxygen depletion can reduce the population of indigenous fish and other oxygen-consuming organisms. Nitrogen nutrients from wastewater have also been linked to ocean “red tides” that poison fish and cause illness in humans. Lastly, nitrogen nutrients in drinking water may contribute to miscarriages and is known to be the cause of a serious illness in infants called “Blue Baby Syndrome”. Of the nitrogen nutrients, nitrates cause the greatest problem.
Accordingly, it is important for aerobic wastewater treatment system to produce treated wastewater which, to the extent possible, is nitrogen nutrient poor and, in particular, contains the minimum possible amount of nitrates.
It is known that there are denitrifying bacteria that can convert dissolved nitrate into harmless nitrogen gas. For denitrifying bacteria to work, several things are required: (1) a source of energy, e.g., organic material, (2) an anoxic environment (one with little to no dissolved oxygen present), (3) nitrates, and (4) efficient mixing and residence time.
To deal with the problem of producing nitrogen nutrient and more particularly nitrate-free treated wastewater from aerobic wastewater treatment plants, it has been proposed to recycle a portion of the effluent from the pump tank to the pre-treatment tank. Although in the pretreatment tank there is sufficient energy available in the form of organic matter, and nitrogen nutrients are plentiful, it is difficult to ensure that the system is anoxic. Further, the mixing/residence time between the denitrifying bacteria and the recycled nitrates are in question. In any event, it has generally been accepted that this method results in reduced nitrogen nutrients, particularly nitrate content in the treated wastewater which is removed from the system.
A typical residential aerobic wastewater treatment plant has a treatment capacity of 500 gallons a day. In the prior art system, recycle of too large of a volume of the treated wastewater from the pump tank to the pretreatment tank can overload the clarifier tank. However, conventionally prior art systems operate in this manner. In this regard, this recycle occurs whenever the pump in the pump tank is discharging the treated wastewater from the pump tank.
Accordingly there is clearly a need for a more efficient and reliable method and/or apparatus for denitrifying treated wastewater from aerobic wastewater treatment systems.